The present invention relates to a pneumatic gripper for screen printing applications, adapted to perform a variable lowering movement, depending on a tensioning stroke thereof.
As is known, for making screen printing operations on fabric materials, the fabric material must be properly tensioned before glueing it to the printing frame.
To that end are used several different apparatus which can be substantially divided into silk tensioning devices and pneumatic grippers.
A silk tensioning apparatus usually comprises a fixed system for tensioning or stretching the fabric material to the desired printing patterns.
Such an apparatus, which mechanically operates, conventionally comprises electric motors, screws and guides, and a plurality of side rubber coated grippers for gripping the fabric material.
The side portions of the apparatus are driven parallel with respect to one another, in order to apply an even deformation to the fabric material and properly stretch it.
The pneumatic grippers, on the other hand, are hand-held tools, having gripping jaws with a width of substantially 15-30 cm designed to grip the fabric material and arrange it about the frame of the screen printing assembly.
The grippers are pneumatically driven and apply to the fabric a set tension level, without deforming it.
The grippers, which are not coupled to one another, can be properly arranged re-combined during the tensioning operation to fit to the screening frame size.
The main advantages of the pneumatic grippers are their very simple construction and low cost.
Said pneumatic grippers, in particular, can be further divided into single movement or effect grippers and dual-movement grippers.
The single movement grippers can be displaced only on a single surface of the fabric material.
Said grippers are very simple construction-wise, but have the drawback that the fabric material contacts the screen printing frame at the start of the operation, while during the stretching operation, the fabric is caused to slide on the screen-printing frame.
The dual-movement grippes, on the other hand, are comparatively complex construction-wise, and stretch the fabric by holding it raised from the screen printing frame, to cause said fabric material to contact the screen printing frame after a further independent lowering displacement.
Typically, both the above disclosed movements are performed pneumatically.
As is further known, a screen printing gripper must usually operate on a lot of different fabric materials of very different elongation and strength characteristics.
Moreover, to the above it is to be further added that pneumatic grippers are not stopped at their end of stroke positions, but only as the desired tension of the fabric being processed has been achieved.
The fabric tension or stretching values are typically included in a range from 20 to 60 N/cm, and the maximum width operating strokes of the grippers are of substantially 100-150 mm.
From an operation standpoint, a dual-movement gripper must meet different requirements which will be thereinbelow illustrated.
At first, it is necessary to protect the gripper from performing overturning movements.
In fact the overturning torque determined by the tensioning force, due to the level difference between the fabric material plane and screen printing frame, is balanced by a stabilizing torque of the weight force with respect to the front end portion of the fabric.
Because of practical weight and length limitations, and upon having set the designed fabric material tension or stretch value, the distance between the fabric material and screen printing frame planes cannot exceed a set limit (typically 15-20 mm), which affects, in a same degree, several operating parameters.
Different screen printing frames have a tapering cross-section and, accordingly, the fabric material lowering movement must be amplified to cause the fabric to properly bear on the top inclined face of the screen frame, to allow the fabric material to be suitably glued through the overall engaged width thereof.
In some cases, a slightly downward directed pulling movement is selected, to facilitate a proper bearing of the fabric material and reduce to suitable values the lowering operating stroke.
The value of the angle on the cross-sections of typically used screen printing frames is smaller than 6-7xc2x0.
The gripper, as it is lowered, must be able of inclining the fabric material with at least the mentioned angle; otherwise, the fabric material would be supported in an unproper manner, as schematically shown in FIG. 10.
The lowering operating stroke, has, in conventional grippers, a substantially constant value.
This feature, together with comparatively high tensioning strokes, would necessarily involve limited lowering angles and a consequent unproper bearing of the fabric material.
Due to the limitations on the distance of the fabric from the screen printing frame (for preventing any overturning from occurring), and since, to favor a proper bearing, an inclined lowering movement is frequently adopted, it can occur that, as a gripper is withdrawn, the fabric material contacts the screen printing frame, thereby causing uneven tension forces, deriving from friction.
Moreover, in a case of a worn screen printing frame, or a screen printing frame with glue residue therein, the fabric material could be undesirably broken.
This problem frequently occurs in grippers having a comparatively long operating stroke.
FIG. 11 schematically illustrates an anomalous contact between the fabric material and screen printing frame.
During the screen printing operation, the tension force is usually calibrat6ed by directly measuring the fabric material by a suitable measuring instruments (the so-called xe2x80x9ctensiometerxe2x80x9d).
Upon having achieved the proper operating tension, the fabric material is lowered with a very high speed, without any possibility of correcting the end tension.
Accordingly, it is important that the lowering movement does not generate anomalous tension variations in the fabric material.
In actual practice, this means that the fabric portion between the gripper jaws and screen printing frame must be held at the same length both at the top or up position, schematically indicated by A in FIG. 12, and in the bottom or down position, schematically indicated by B in FIG. 12.
Since the fabric material has a substantially elastic nature, to equal deformations equal efforts or strain would correspond, thereby holding the applied tension.
The above disclosed three requirements must meet contrasting needs which, for high performance grippers (50-60 N/cm, and a stroke  greater than 100 mm) do not allow designing tolerances for the designed.
In fact, to obtain a good bearing of the fabric, it is necessary to incline with a very great inclined angle the fabric during the tensioning movement thereof.
However, this would require, to prevent any anomalous contacts between the fabric and screen printing frame, to greatly increase the starting distance between the fabric and screen printing frame, which would be impossible in order not to cause the gripper to loose its stability properties.
Since all the available screen printing grippers carry out a constant-stroke lowering movement, or a decreasing stroke as the gripper jaws (i.e. the rear hinged swinging portion) are withdrawn it could be stated that for said gripper exit maximum limit tensioning stroke, beyond which the operation of the grippers would become uneven.
Accordingly, the aim of the present invention is to provide such a gripper which overcomes the above mentioned problems.
Within the scope of the above mentioned aim, a main object of the present invention is to provide such a screen printing pneumatic gripper, of a dual-movement type, having an uneven (i.e. variable) lowering stroke, and designed for assuming higher values for greater tensioning strokes, thereby allowing the fabric material to be lowered with an angle sufficient to allow, in all operating cases, the fabric material to be properly supported, even on tapering cross-section frames.
Another object of the present invention is to provide such a gripper which can be industrially made with a simplified series production.
According to one aspect of the present invention, the above mentioned aim and objects, as well as yet other objects, which will become more apparent hereinafter, are achieved by a screen printing gripper comprising a screen printing gripper body supporting a gripping assembly supporting, in turn, a gripping element to grip an edge portion of a fabric material to be tensioned on a frame, characterized in that said screen printing gripper further comprises a bottom abutment cam designed for following a variable lowering stroke depending on a withdrawn stroke of the gripping element, to provide a variable lowering movement varying depending on a tensioning stroke of said fabric material.